The highest temperature attainable by the addition of oxygen, in comparison to the normal temperature of the exhaust gas, is measured with the aid of a thermal sensor. This apparatus has the disadvantage that not only carbon monoxide (CO) but the other still combustible components remaining after combustion, such as unburned hydrocarbons, are also converted at the catalytically active thermal sensor, influencing the resultant heat level. The sensor accordingly indicates only an inexact value for the toxic exhaust component CO. In particular, characteristic points of the CO spectrum cannot then be ascertained and utilized for closed-loop control purposes.
It is also known to control a quantity of recirculated exhaust gas in open-loop fashion in an engine having external mixture formation. In addition to other variables, the fuel-air ratio, for instance, which is ascertainable via the CO content, is taken into consideration.